Secure chip identification using random threshold voltage variation in a field effect transistor structure as a physically unclonable function

ABSTRACT

A semiconductor structure may include one or more metal gates, one or more channels below the one or more metal gates, a gate dielectric layer separating the one or more metal gates from the one or more channels, and a high-k material embedded in the gate dielectric layer. Both the high-k material and the gate dielectric layer may be in direct contact with the one or more channels. The high-k material may provide threshold voltage variation in the one or more metal gates. The high-k material is a first high-k material or a second high-k material. The semiconductor structure may only include the first high-k material embedded in the gate dielectric layer. The semiconductor structure may only include the second high-k material embedded in the gate dielectric layer. The semiconductor structure may include both the first high-k material and the second high-k material embedded in the gate dielectric layer.

BACKGROUND

The present invention relates generally to a semiconductor structure anda method of forming the same. More particularly, the present inventionrelates to secure chip identification using random threshold voltage(V_(th)) variation in a field effect transistor (FET) structure as aphysically unclonable function (PUF).

A PUF is a physically-defined “digital fingerprint” that serves as aunique identity for a semiconductor device, such as, for example, amicroprocessor. The PUF is a physical entity embodied in a physicalstructure. The PUF is based on a unique physical variation which mayoccur naturally during semiconductor manufacturing. The PUF may beimplemented in an integrated circuit. The PUF may also be sued inapplications with high security requirements, such as, for example,cryptography.

SUMMARY

According to one embodiment of the present invention, a semiconductorstructure is provided. The semiconductor structure may include one ormore metal gates, one or more channels below the one or more metalgates, a gate dielectric layer separating the one or more metal gatesfrom the one or more channels, and a high-k material embedded in thegate dielectric layer. Both the high-k material and the gate dielectriclayer may be in direct contact with the one or more channels. The high-kmaterial may provide threshold voltage variation in the one or moremetal gates. The high-k material is a first high-k material or a secondhigh-k material. The semiconductor structure may include only the firsthigh-k material embedded in the gate dielectric layer. The semiconductorstructure may include only the second high-k material embedded in thegate dielectric layer. The semiconductor structure may include both thefirst high-k material and the second high-k material embedded in thegate dielectric layer.

According to another embodiment of the present invention, asemiconductor structure is provided. The semiconductor structure mayinclude one or more gate regions of one or more field effecttransistors. The one or more gate regions may be above one or morechannels. The semiconductor structure may include a metal gate separatedfrom the one or more channels by a gate dielectric layer, a high-kmaterial embedded in the gate dielectric layer, and an interlayerdielectric. Both the high-k material and the gate dielectric layer maybe in direct contact with the one or more channels. The interlayerdielectric may surround the one or more gate regions. The high-kmaterial may be a first high-k material or a second high-k material. Thesemiconductor structure may include only the first high-k materialembedded in the gate dielectric layer. The semiconductor structure mayinclude only the second high-k material embedded in the gate dielectriclayer. The semiconductor structure may include both the first high-kmaterial and the second high-k material embedded in the gate dielectriclayer. The high-k material may provide a threshold voltage variation inthe one or more metal gates.

According to another embodiment of the present invention, a method isprovided. The method may include forming one or more gate regions abovea channel, randomly nucleating a high-k material in the one or more gateregions, and depositing a gate dielectric layer in each of the one ormore gate regions. Randomly nucleating the high-k material in the one ormore gate regions may facilitate a random threshold voltage within theone or more gate regions. The random threshold voltage within the one ormore gate regions may be used as a physically unclonable function. Theone or more gate regions may be surrounded by a gate spacer. The gatedielectric layer may cover the entire surface of the channel. The secondhigh-k material may partially cover the surface of the channel. Thehigh-k material may be a first high-k material or a second high-kmaterial. The method may include embedding only the first high-kmaterial in the gate dielectric layer. The method may include embeddingonly the second high-k material in the gate dielectric layer. The methodmay include embedding both the first high-k material and the secondhigh-k material in the gate dielectric layer. The method may includeforming a first source drain and a second source drain, depositing ametal gate on top of the gate dielectric layer, depositing an interlayerdielectric around the one or more gate regions, and forming a firstsource drain contact and a second source drain contact. The first sourcedrain and the second source drains may be separated from the one or moregate regions by the gate spacer. The interlayer dielectric may be on topof the first source drain and the second source drain.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintend to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which:

FIG. 1 is a cross section view illustrating a fin and a buried oxidearranged on a substrate according to an exemplary embodiment;

FIG. 2 is a cross section view illustrating a dummy gate and a gatespacer arranged on the fin according to an exemplary embodiment;

FIG. 3 is a cross section view illustrating a first and second sourcedrains and an interlayer dielectric according to an exemplaryembodiment;

FIG. 4 are cross section views illustrating multiple gate regions withtrenches formed to expose top surfaces of the fins according to anexemplary embodiment;

FIG. 5 are cross section views illustrating multiple gate regions with afirst high-k material according to an exemplary embodiment;

FIG. 6 are cross section views illustrating multiple gate regions with asecond high-k material according to an exemplary embodiment;

FIG. 7 are cross section views illustrating multiple gate regions with agate dielectric layer according to an exemplary embodiment;

FIG. 8 are cross section views illustrating multiple gate regions with agate metal and contacts according to an exemplary embodiment; and

FIG. 9 are top views of gate regions illustrating random nucleation offirst and second high-k materials according to an exemplary embodiment.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiment set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

Embodiments of the present invention relate generally to a semiconductorstructure and a method of forming the same. More particularly, thepresent invention relates to secure chip identification using randomthreshold voltage (V_(th)) variation in a field effect transistor (FET)structure as a physically unclonable function (PUF).

Semiconductor supply chain integrity is a vital security need,particularly for national defense. The loss of trusted foundries in theUS has underscored the need to verify that chips installed in criticalsystems are authentic. For example, phony components, if substituted forreal ones, can contain malicious circuitry enabling loss offunctionality or snooping. As a result, there exists a need to fabricatea semiconductor device that may be identified as secure.

Many semiconductor devices use PUFs as a security measure. A PUF is aphysically-defined “digital fingerprint” that serves as a uniqueidentity for a semiconductor device, such as, for example, amicroprocessor. The PUF is a physical object that for a given input andconditions (challenge), may provide a physically-defined “digitalfingerprint” output (response) that serves as a unique identifier. ThePUF is based on a unique physical variation which may occur naturallyduring semiconductor manufacturing. The PUF may be implemented in anintegrated circuit. The PUF may also be used in applications with highsecurity requirements, such as, for example, cryptography.

Silicon PUFs are increasingly used as a hardware root of trust and anentropy source for cryptography applications. In those applications, thereliability of a PUF output is key to a successful implementation. Bothweak and strong PUFs obtain output by amplifying analog signals fromphysical properties on integrated circuit blocks. The analog signal maybe a propagation delay, a ring oscillator, a time-controlled oxidebreakdown, or a V_(th) of SRAM transistor. These physical measurementsare by nature sensitive to environmental conditions, such as, forexample, temperature, operating voltage, thermal/interface noise oftransistors, process corners, and aging. As a result, it is difficult toobtain a stable PUF output without taking additional stabilization anderror-correction techniques, such as, for example, temporal majorityvoting (TMV), pre-burning on PUF bits for end-of-life (EOL) predictionand reliability screening, masking algorithms, as well as leveragingparity bits for an Error-Correcting-Code (ECC). As a result, thereexists a need to fabricate a semiconductor device with a stable PUFoutput.

Embodiments of the present invention provide a structure and a method toform a random V_(th) variation in a gate structure of a FET. The FET maybe a planar FET, a nanosheet FET, a vertical FET, a nanowire FET, a finFET, or any other FET or semiconductor device having a gate structure.The random V_(th) is used as the PUF and is accomplished by differenthigh-k materials randomly deposited in multiple gate regions of thedevice. FIGS. 1-8 illustrate a method of making a FET structure with arandom V_(th) variation as the PUF. FIG. 9 illustrates top cross sectionviews of gate regions with randomly distributed high-k materials thatenable different V_(th) to be generated.

Referring now to FIG. 1 , a structure 100 is shown, in accordance withan embodiment. The structure 100 may include a substrate 102, a buriedoxide 104, and a fin 106. The substrate 102 may include one or moresemiconductor materials. Non-limiting examples of suitable substrate 102materials may include Si (silicon), strained Si, Ge (germanium), SiGe(silicon germanium), Si alloys, Ge alloys, III-V materials (e.g., GaAs(gallium arsenide), InAs (indium arsenide), InP (indium phosphide), orIndium Gallium Arsenide (InGaAs), II-VI materials (e.g., CdSe (cadmiumselenide), CdS (cadmium sulfide), CdTe (cadmium telluride)), or anycombination thereof. In an embodiment, the substrate 102 may includesilicon.

The buried oxide 104 is arranged on the substrate 102. The buried oxide104 may be an oxide layer, such as, for example, silicon oxide, buriedin a silicon wafer. The fin 106 is arranged on a top surface of theburied oxide 104. The fin 106 may be made of silicon. In an embodiment,the fin 106 may be a silicon-on-insulator fin. In an alternativeembodiment, the fin 106 may be a bulk fin. Further, it should beappreciated that even though a silicon-on-insulator fin is illustrated,embodiments of the present invention may be implemented on any CMOSdevice or a transistor device that includes a gate structure.

Referring now to FIG. 2 , the structure 100 with a dummy gate 108, agate spacer 110, a first source drain 112, and a second source drain 114is shown, in accordance with an embodiment. A sacrificial gate materialmay be deposited on the top surface of the fin 106. The sacrificial gatematerial may then be patterned to form the dummy gate 108.

The dummy gate 108 may be formed from the sacrificial gate material,such as, for example, amorphous silicon (a-Si) or polycrystallinesilicon (polysilicon). The sacrificial material may be deposited by adeposition process, including, but not limited to, physical vapordeposition (PVD), chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition (PECVD), inductively coupled plasma chemicalvapor deposition (ICP CVD), or any combination thereof. The sacrificialmaterial forming the dummy gate 108 may have a thickness of about 50 nmto about 250 nm, or from about 100 nm to about 200 nm.

Once the dummy gate 108 is formed, the gate spacer 110 is thenconformally deposited on the dummy gate 108. The gate spacer 110 is thenpatterned to remove portions of the gate spacer 110 that are on the topsurface of the fin 106. As a result, the gate spacer 110 surrounds thedummy gate 108. The gate spacer 110 is on the top surface and thesidewalls of the dummy gate 108.

The gate spacer 110 may include an insulating material, such as, forexample, silicon dioxide, silicon nitride, SiOCN, or SiBCN. Othernon-limiting examples of materials for the gate spacer 110 may includedielectric oxides (e.g., silicon oxide), dielectric nitrides (e.g.,silicon nitride), dielectric oxynitrides, or any combination thereof.The gate spacer 110 materials are deposited by a deposition process, forexample, chemical vapor deposition (CVD) or physical vapor deposition(PVD). The gate spacer 110 may have a thickness of about 3 to about 15nm, or of about 5 to about 8 nm. The gate spacer 110 insulates the dummygate 108 from other materials formed or deposited around the dummy gate108. Further, the gate spacer 110 separates the dummy gate 108 from thefirst and second source drains 112, 114.

Once the gate spacer 110 is deposited, an epitaxial layer is grown onthe surface of the fin 106 to form the first source drain 112 and thesecond source drain 114. The first and second source drains 112, 114 maybe grown using a suitable growth process, for example, chemical vapordeposition (CVD) (low pressure (LP) or reduced pressure chemical vapordeposition (RPCVD), vapor-phase epitaxy (VPE), molecular-beam epitaxy(MBE), liquid-phase epitaxy (LPE), metal organic chemical vapordeposition (MOCVD), or other suitable processes.

The sources for the epitaxial layer material may be, for example,silicon, germanium, or a combination thereof. The gas source for thedeposition of epitaxial semiconductor material may include asilicon-containing gas source, a germanium-containing gas source, or acombination thereof. For example, an epitaxial silicon layer may bedeposited from a silicon gas source that is selected from the groupconsisting of silane, disilane, trisilane, tetrasilane,hexachlorodisilane, tetrachlorosilane, dichlorosilane, trichlorosilane,and combinations thereof. An epitaxial germanium layer can be depositedfrom a germanium gas source that is selected from the group consistingof germane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. An epitaxial silicongermanium alloy layer can be formed utilizing a combination of such gassources. Carrier gases like hydrogen, nitrogen, helium, and argon can beused.

The first and second source drains 112, 114 may be in-situ doped with ap-type dopant or an n-type dopant. For example, for a p-type FET, thefirst and second source drains 112, 114 may be made of silicon germaniumand in-situ doped with a p-type dopant, such as, for example, boron. Foran n-type FET, the first and second source drains 112, 114 may be madeof silicon and in-situ doped with an n-type dopant, such as, forexample, phosphorus or arsenic.

Referring now to FIG. 3 , the structure 100 with an interlayerdielectric (ILD) 116 is shown, in accordance with an embodiment. Oncethe first and second source drains 112, 114 are formed, the ILD 116 maybe deposited on to the top surfaces of the first and second sourcedrains 112, 114 such that top surface of the ILD 116 is substantiallyflush with the top surface of the gate spacer 110 (not illustrated). TheILD 116 surrounds the gate spacer 110. The ILD 116 may be formed from,for example, a low-k dielectric material (with k<4.0), including but notlimited to, silicon oxide, spin-on-glass, a flowable oxide, a highdensity plasma oxide, borophosphosilicate glass (BPSG), or anycombination thereof.

The ILD 116 is deposited by a deposition process, including, but notlimited to CVD, PVD, plasma enhanced CVD, evaporation, chemical solutiondeposition, or like processes. After the ILD 116 is deposited, aplanarization process, such as, for example, a CMP process, may be usedto remove excess ILD 116 from the top surface of the structure 100. TheCMP process may also be used to remove portions of the gate spacer 110directly above the dummy gate 108. Removing the gate spacer 110 directlyabove the dummy gate 108 exposes the top surface of the dummy gate 108such that the dummy gate 108 may be removed in subsequent process steps.

Referring now to FIG. 4 , multiple structures 100 with trenches 118 areshown, in accordance with an embodiment. It should be appreciated thateven though three structures 100 are illustrated, embodiments of thepresent invention contemplate forming a plurality of structures 100 on asingle substrate wafer. Each structure 100 includes a gate regionsurrounded by the ILD. FIG. 4 illustrates three gate regions, gateregion 1, gate region 2, and gate region 3, for the purpose ofillustrating that there are more than one gate region on a silicon wafersince there are more than one structure 100 on the silicon wafer.

An etch process, such as, for example, a wet etch process may beperformed to remove the dummy gates 108 from each gate region of thestructures 100 until the top surfaces of the fins 106 are exposed,thereby forming the trenches 118. Each trench 118 may extend from thetop surface of the gate spacer 110 to the top surface of the fin 106.The etch process removes the dummy gate 108 material selective to thefin 106 material. However, the tech process does not remove the gatespacer 110. As a result, the gate spacer 110 separates the first and thesecond source drains 112, 114 from the gate region.

Referring now to FIG. 5 , three gate regions of structures 100 areshown, in accordance with an embodiment. Using a timed atomic layerdeposition process, the first high-k material 120 is randomly depositedinto the trenches 118 of the structures 100. The first high-k material120 is deposited onto the surface of the exposed fins 106 (in the gateregion). A deposition time very close to the nucleation time is chosento facilitate random nucleation. Due to said nucleation variation, someof the first high-k material 120 may deposit randomly at some gateregions, such that the first high-k material 120 may partially cover thesurface of the exposed fin 106 (in the gate region). As a result, somegate regions may have least a portion of the top surface of the fin 106remaining exposed after the formation of the first high-k material 120.Further, not all gate regions of structures 100 may have the firsthigh-k material 120. As is illustrated, gate regions 1 and 3 include thefirst high-k material 120, whereas gate region 2 does not. In addition,the randomness of nucleation cannot be repeated. As a result, conductinganother deposition of the first high-k material 120 may not yield thesame results with gate regions 1 and 3 including the first high-kmaterial 120. It should be appreciated that a typical substrate wafermay include thousands, even millions, of gate regions. Due to the randomnucleation, each substrate wafer may have millions of gate regions withdifferent first high-k material 120 nucleation. As a result, no twowafers will have all of its gate regions identically nucleated with thefirst high-k material 120.

The first high-k material 120 may be made of material such as aluminumoxide (Al₂O₃). Other materials, such as, for example, lutetium oxide(Lu₂O₃), yttrium oxide (Y₂O₃), magnesium oxide (MgO), and other rareearth oxides, may also be used as the first high-k material 120. Thecomposition, deposition process, and post-deposition treatment of thefirst high-k material 120 may have a different effect on the thresholdvoltage (V_(th)) of the structure 100, depending on whether thestructure 100 is a pFET or an nFET, the electrical properties of theinterface between the high-k material 120 and the top surface of the fin106 (including interfacial oxide formation during or after high-kmaterial deposition), the composition of the gate metal, the polarity ofthe distributed charges through the high-k dielectric and at the high-kdielectric/fin interface, as well as other device and process parameterswell known in the art.

Referring now to FIG. 6 , three gate regions of structures 100 with asecond high-k material 122 are shown, in accordance with an embodiment.Using a timed atomic layer deposition process, the second high-kmaterial 122 is deposited into the trenches 118 of the structures 100.The second high-k material 122 is deposited the surface of the exposedfins 106 (in the gate region). Just like the first high-k material 120,the second high-k material 122 may randomly nucleate in different gateregions, thus partially covering the surface of the exposed fins 106 (inthe gate region). As a result, some gate regions may have least aportion of the surface of the exposed fin 106 (in the gate region)remaining exposed after the formation of the second high-k material 122.As a result, some gate regions may include both the first high-kmaterial 120 and the second high-k material 122, whereas some gateregions may either have the first high-k material 120 or the secondhigh-k material 122. There may also be gate regions with neither thefirst high-k material 120 nor the second high-k material 122. Asdescribed above with reference to the first high-k material 120, thesecond high-k material 122 also nucleates randomly within the millionsof gate regions on a substrate wafer.

During nucleation, the second high-k material 122 may overlap the firsthigh-k material 120. The second high-k material 122 may also be adjacentto the first high-k material 120. The second high-k material 122 eitherbe in contact with the first high-k material 120 or not contacting thefirst high-k material 120 even if both the first and second high-kmaterials 120, 122 are present in the same gate region.

The second high-k material 122 may be made of material such as lanthanumoxide (La₂O₃), or other materials, such as, for example, lutetium oxide(Lu₂O₃), yttrium oxide (Y₂O₃), magnesium oxide (MgO), and other rareearth oxides. Similar to the first high-k material 120, the secondhigh-k material 122 may have a different effect on V_(th), depending onits composition, as well as various other material, device and processparameters as known in the art. For example, the first high-k material120 may increase the V_(th) while the second high-k material 122 maydecrease the V_(th) and vice versa; or they may both increase ordecrease the V_(th).

Referring now to FIG. 7 , three gate regions of structures 100 with agate dielectric layer 124 are shown, in accordance with an embodiment.Using an atomic layer deposition process, the gate dielectric layer 124is conformally deposited into the trenches 118 of the structures 100,along the bottom and sidewalls of the trenches 118. The gate dielectriclayer 124 is deposited into the trenches 118 of all of the open gateregions. The gate dielectric layer 124 is made with high-k dielectricmaterial. Non-limiting examples of suitable materials for the gatedielectric layer 124 include oxides, nitrides, oxynitrides, silicates(e.g., metal silicates), aluminates, titanates, nitrides, or anycombination thereof. Examples of high-k materials include, but are notlimited to, metal oxides such as hafnium oxide (HfO₂). The portion ofthe fin 106 under the gate dielectric layer 124 may also be referred toas a channel.

Referring now to FIG. 8 , three gate regions of structures 100 with ametal gate 126 are shown, in accordance with an embodiment. Once thegate dielectric layer 124 is deposited along the sidewalls and bottom ofthe trenches 118, the trenches 118 are filled with a conductive metal toform the metal gates 126. Non-limiting examples of the conductivematerial that forms the metal gates 126 may include aluminum (Al),platinum (Pt), gold (Au), tungsten (W), titanium (Ti), or anycombination thereof. The conductive metal may be deposited by a suitabledeposition process, for example, chemical vapor deposition,plasma-enhanced chemical vapor deposition, physical vapor deposition,plating, thermal or e-beam evaporation, and sputtering.

In addition to the formation of the metal gates 126, the structures 100also undergo further processing to form contacts, including a first andsecond source drain contacts 128, 130, illustrated in gate region 1 ofFIG. 8 . The first and second source drain contacts 128, 130 extendthrough the ILD 116 to the first and second source drains 112, 114,respectively and are formed within trenches. To remove the ILD 116 andform the contact trenches, a resist, such as a photoresist, may bedeposited and patterned. An etch process, such as reactive ion etch, maybe performed using the patterned resist as an etch mask to remove theILD 116 until the first and second top source drains 112, 114 areexposed. The contact trenches are filled with a conductive material or acombination of conductive materials to form the first and second sourcedrain contacts 128, 130. The conductive material filling may be aconductive metal, for example, aluminum (Al), platinum (Pt), gold (Au),tungsten (W), titanium (Ti), or any combination thereof. The conductivematerial may be deposited by a suitable deposition process, for example,chemical vapor deposition, plasma enhanced chemical vapor deposition,physical vapor deposition, plating, thermal or e-beam evaporation, orsputtering. A planarization process, for example, chemical mechanicalplanarization, is performed to remove any conductive material from thesurface of the ILD 116.

The resultant structures 100, as illustrated in FIG. 8 , include gateregions that may contain different combinations of the first and secondhigh-k materials 120, 122. For example, some gate regions may includeboth the first and second high-k materials 120, 122, as is illustratedin gate region 1. Some gate regions may only include the first high-kmaterial 120, as is illustrated in gate region 2, or the second high-kmaterial 122, as is illustrated in gate region 3. In addition, there maybe gate regions, not illustrated, which do not contain either the firsthigh-k material 120 or the second high-k material 122. However, all gateregions include the gate dielectric layer 124. All gate regions may alsoinclude the metal gate 126.

Having the first and/or the second high-k materials 120, 122 in certaingate regions of a circuit device produces or facilitates differentthreshold voltages in these gate regions. The different thresholdvoltages are accomplished by the use of the first and the second high-kmaterials 120, 122 that have different chemical compositions, incombination with the gate dielectric layer 124. For example, the firsthigh-k material 120 may be referred to as A, the second high-k material122 may be referred to as B and the gate dielectric layer 124 may bereferred to as C. As described herein above, all gate regions include C.Some gate regions include C only. Some gate regions have a combinationof C and A, some gate regions have a combination of C and B, and somegate regions have a combination of C, A, and B, where A and B may benext to each other, or may even overlap. As a result, the differentthreshold voltages may provide a random set of transistors in a givenchip. The randomness is accomplished through the random nucleation ofthe first and/or the second high-k materials 120, 122 in the gateregions of structures 100. As described above, nucleation within thegate regions is random such that each substrate wafer may have differentcombination of the first and/or second high k-materials 120, 122. As aresult, some gate regions include both the first and the second high-kmaterials 120, 122, some gate regions include only one of the high-kmaterials, whereas some gate regions include neither the first nor thesecond high-k materials 120, 122. In addition, a typical substrate wafermay include millions of gate regions. Due to the randomness ofnucleation of the high-k materials, no two substrate wafers will havethe same deposition pattern of the first and second high-k materials120, 122 in each of the gate regions of both wafers. As a result, eachgate region of each substrate wafer has a particular threshold voltage.These threshold voltages may be used as PUFs to identify each chip.

Referring now to FIG. 9 , top views of gate regions illustrating randomnucleation of first and second high-k materials is shown, in accordancewith an embodiment. As described herein above, the V_(th) refers to the“effective” or “extrapolated” threshold voltage of the transistor. Forexample, an effective threshold voltage may refer to the minimum voltagerequired to turn on a transistor and have current (for example, ameasurable or detectable current) flow from the first source drain 112to the second source drain 114, or vice versa. Having either the first,the second, or both, high k-materials 120, 122 nucleating in the gateregions provides variation in the effective threshold voltage due to therandomness of the nucleation process.

The cross-sections in the top row illustrate the first high-k material120 and the second high-k material 122 extending from top to bottom. Inthis exemplary embodiment, the composition of the high-k materials andthe device and process parameters are such that the portion of thetransistor channel covered by the second high-k material 122 has a“locally” higher threshold voltage than the portion of the transistorchannel covered by the first high-k material 120. As a result, theaddition of the second high-k material 122 increases the effectivethreshold voltage of the transistor. This increase in the effectivethreshold voltage of the transistor is most significant when the secondhigh-k material 122 spans the entire channel width of the transistor asillustrated in the top row. This is because, when a gate voltage of thetransistor is lower than the local threshold voltage associated with thechannel region covered by the second high-k material 122 but higher thanthe local threshold voltage associated with the channel region coveredby the first high-k material 120, the presence of the second high-kmaterial 122 across the entire channel width effectively blocks currentflow between the two source drains 112, 114; whereas in the embodimentsillustrated in the bottom row, there still exists a path for currentflow between the two source drains 112, 114 through the locally lowerthreshold voltage region of the channel covered by the first high-kmaterial 120. Therefore, the presence of the second high-k material 122in the embodiments illustrated in the bottom row may result in arelatively small or negligible increase in the effective thresholdvoltage compared to the embodiments illustrated in the top row. However,the presence of the second high-k material 122 in the embodimentsillustrated in the bottom row can reduce the ON current of thetransistor by reducing the effective channel width available for currentflow. The presence of the second high-k material 122 in the embodimentsillustrated in the top row can also reduce the ON current of thetransistor by virtue of increasing the effective threshold voltage ofthe transistor.

The peripheral circuitry may detect the transistor-to-transistorvariability in the effective threshold voltage, thetransistor-to-transistor variability in the ON current (for a given gatebias or a range of gate biases), or a combination of both. As statedabove, threshold-voltage sensing circuitry are most effective indetecting variations arising from random nucleation in the embodimentsshown in the top row of FIG. 9 ; whereas current sensing circuitry canbe effective in detecting variations arising from random nucleation inthe embodiments illustrated in both rows. The threshold voltage sensingperipheral circuitry may be based on, for example, switched capacitors,or other circuit configurations known in the art. The current sensingcircuitry may be based on, for example, sense amplifiers or othercircuit configurations known in the art. Both methods of detectingtransistor-to-transistor variability are within the scope of thisinvention.

Embodiments of the present invention describe using the nucleationeffect of atomic layer deposition of high-k materials in the gateregions to form random V_(th) of any field effect transistor structureor other structures that include a gate and a gate dielectric layer. Therandomness of nucleation of the first and second high-k materials 120,122 allows for the V_(th) to be used as a PUF, thus providing chip/waferidentification and authenticity.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiment, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A semiconductor structure comprising: one or moremetal gates; one or more channels below the one or more metal gates; agate dielectric layer separating the one or more metal gates from theone or more channels; and a first high-k material and a second high-kmaterial embedded in the gate dielectric layer, wherein the first high-kmaterial, the second high-k material, and the gate dielectric layer arein direct contact with the one or more channels.
 2. The semiconductorstructure of claim 1, wherein the first high-k material and the secondhigh-k material are configured to provide a threshold voltage variationin the one or more metal gates.
 3. The semiconductor structure of claim1, wherein the first high-k material is made of aluminum oxide and thesecond high-k material is made of lanthanum oxide.
 4. A semiconductorstructure comprising: one or more gate regions of one or more fieldeffect transistors, the one or more gate regions is above one or morechannels; a metal gate, the metal gate is separated from the one or morechannels by a gate dielectric layer; a first high-k material and asecond high-k material embedded in the gate dielectric layer, whereinthe first high-k material, the second high-k material, and the gatedielectric layer are in direct contact with the one or more channels;and an interlayer dielectric, the interlayer dielectric surrounding theone or more gate regions.
 5. The semiconductor structure of claim 4,wherein the first high-k material and the second high-k material areconfigured to provide a threshold voltage variation in the one or moremetal gates.
 6. The semiconductor structure of claim 4, wherein thefirst high-k material is made of aluminum oxide and the second high-kmaterial is made of lanthanum oxide.
 7. A semiconductor structurecomprising: one or more metal gates; one or more channels below the oneor more metal gates; a gate dielectric layer separating the one or moremetal gates from the one or more channels; and a first high-k materialand a second high-k material embedded in the gate dielectric layer,wherein the first high-k material, the second high-k material, and thegate dielectric layer are in direct contact with the one or morechannels, and wherein the high-k material partially covers a surface ofthe one or more channels such that at least a portion of the one or morechannels remains exposed.
 8. The semiconductor structure of claim 7,wherein the first high-k material and the second high-k material areconfigured to provide a threshold voltage variation in the one or moremetal gates.
 9. The semiconductor structure of claim 7, wherein thefirst high-k material is made of aluminum oxide and the second high-kmaterial is made of lanthanum oxide.